Nonlinear Optical Polymers: Challenges and Opportunities in Photonics

In polymer structures, highly correlated virtual excitations of the π-electrons are responsible for the exceptionally large nonresonant nonlinear optical responses observed. Extremely large resonant nonlinear optical responses are also achievable in certain π-electron systems, which can be treated as optical Bloch systems. In addition to their obvious scientific importance, these large optical nonlinearities potentially make possible the implementation of powerful, new nonlinear optical devices and systems. After a description of nonlinear optical processes in polymers, two examples are presented. First, saturable absorption and optical bistability in ultrathin organic polymer films are described, illustrative of resonant third order processes. Saturable absorption studies of glassy polymer films consisting of quasi two-dimensional conjugated disc-like structures of silicon naphthalocyanine demonstrate that on-resonance the system behaves as an optical Bloch system with a linear absorptivity coefficient α0 of 1 × 105 cm−1 and an intensity dependent refractive index n2 of 1 × 104 cm2/kW in the wavelength range of standard laser diodes. A resonant nonlinear optical response of π-electron excitations provides the nonlinear interaction essential to the onset of bistability. Electronic absorptive optical bistability is observed on a nanosecond time scale in a nonlinear Fabry-Perot interferometer employing the saturably absorbing naphthalocyanine film as the nonlinear optical medium. As a second example, the nonresonant second order process of linear electrooptic effects in poled polymer films, is discussed. For such a second order nonlinear optical process, the broken global centrosymmetry is achieved by electric field poling of a thin polymer film. With high electrooptic coefficients of 10–50 pm/V and low dielectric constants of 3–4, poled polymers have potentially great advantages over inorganic crystals as electrooptic materials. As one device illustration, the application of poled polymers in electrooptic waveguide operations is presented.

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